Stimulus Internet Protocol (IP) phones differ from Time Division Multiplexing (TDM) based phones in that a breach in the communications link to the PBX is not known immediately. Most stimuli based IP phones use a “keepalive” scheme that requires one endpoint to periodically send a message across a control link to the other. The absence of the message when expected, or the absence of a response, inform the endpoint that the control link is no longer active. The state of the art in keepalive schemes is to use Transmission Control Protocol (TCP) layer keepalives, application layer keepalives, which are also known as “heartbeats”, or PINGs. These methods generally entail a messaging burden upon the PBX. The exception is when the PBX uses a multicast or broadcast heartbeat scheme. However, these schemes are problematic in that broadcasts are isolated to specific subnets, and multicast may not reach to other subnets on all networks, as some routers in the field do not support Internet Group Management Protocol (IGMP).
Some networks support a control link loss scheme that requires bidirectional application layer messaging. In this type of scheme, the administrator configures the IP phone poll and the heartbeat time periods to suit the needs of the network. The PBX sends each phone the information in a “Configure Link Management” control message which enables the feature in the phones. The PBX will expect to see a control message every poll period from each IP phone and each IP phone will expect to see a control message from the PBX every heartbeat period. If either side finds that the poll or heartbeat is absent, the control link can be torn down and data structures re-initialized. Alternatively, the phone can re-home to a new PBX.
The size of PBXs continues to grow, with increasing numbers of phones being supported by each PBX. If the control link between the PBX and a phone is lost for any reason, the phone loses its ability to make and receive calls. It is therefore desirable to shorten link loss discovery times in order to ensure that users are provided with timely availability of phone service. Shortening link loss discovery times by increasing the volume and/or frequency of messages sent between the PBX and the IP phones may be cumbersome to the PBX. Therefore, a solution for shortening link loss discovery times while not increasing the volume and/or frequency of messages is desirable.